High speed linac-beam analyzer

ABSTRACT

A device for analyzing a charged particle beam developed by an accelerator device having an RF driving signal is provided. The particle beam passes through a transparent medium, developing light of intensity proportional to the intensity of the particle beam. A photocathode is aligned to detect the light and thereby generate an electron beam of intensity proportional to the intensity of the light. The RF driving signal is coupled via a phase-varying network to an X-axis deflection system and, after a phase shift of 90*, to a Y-axis deflection system. The electron beam is directed through the X-axis and Y-axis deflection system, thereby causing the electron beam to precess about an axis and describe a circular trace in a plane perpendicular to the axis. Means are provided to measure the intensity of the beam along a particular narrow arc of the circular trace as the phase of the RF signal applied to the X and Y deflection systems is varied from 0* to 360*.

United States Patent 91 Johnson [4 1 Mar. 25, 1975 HIGH SPEED LINAC-BEAMANALYZER [75] Inventor: Kenneth W. Johnson, Lockport, Ill.

[21] Appl. No.: 459,824

Primary E.\'aminer.lames W. Lawrence Assistant Examiner-T. N. GrigsbyAttorney, Agent, or Firm-Dean E. Carlson; Arthur A. Churm; Paul A.Gottlieb [57] ABSTRACT 52 us. Cl 250/369 250/361 250/362 tocathode isaligned to detect the light and thereby 250/396 generate an electronbeam of intensity porportional to [51] Int CL Golj 39/18, G01" 21/16 G0H20 the intensity of the light. The RF driving signal is cou- 581 Fieldof Search 250/369 36i 362 379 PM Via a Phasevarying network to X-aXiSdeflec- 6 tion system and, after a phase shift of 90, to a Y-axisdeflection system. The electron beam is directed [56] References Citedthrough the X-axis and Y-axis deflection system, thereby causing theelectron beam to precess about an UNITED STATES PATENTS axis anddescribe a circular trace in a plane perpendicular to the axis. Meansare provided to measure the Cover e intensity of the beam along aparticular narrow arc of gay s the circular trace as the phase of the RFsignal applied 4/1974 l 750/369 to the X and Y deflection system isvaried from 0 to 7 Claims, 4 Drawing Figures OSCILLATOR I 4Z0flMPL/F/Ef? j il it M F1 7 V/YlP/HBLE Had/4 f0"; PMEE REC/ORDER ISHIFTER ea I l i 90 //60 55???? {-25 l l l/ I 26 PATENTEDHARZSISYS SHEET1 2 tmmkgmt mmhmim w w m HIGH SPEED LINAC-BEAM ANALYZER CONTRACTUALORIGIN OF THE INVENTION The invention described herein was made in the'course of, or under, a contract with the UNITED STATES ATOMIC ENERGYCOMMISSION.

BACKGROUND OF THE INVENTION A particle beam analyzer is a tool forexamining the frequency and amplitude characteristics of a beam ofcharged particles developed by an accelerator. For example, the idealbeam developed by a linear accelerator consists ofa series ofsubstantially identical bunches or pulses of charged particles travelingat a speed approaching the speed of light, with the frequency of thegeneration of each bunch by the accelerator corresponding to thefrequency of an RF driving signal. An analyzer should detect thepresence of these pulses and provide means for determining how closelycoincident in time the particles are generated, which provides a measureof the quality of the bunching together of the particles.

Certain types of linear accelerators produce particle beams having afrequency of bunch generation greater than other types requiring aparticle beam analyzer capable of sensitivity to the higher frequencyaccelerators. Thus the analyzer must be able to respond more quickly tothe pulses which comprise the beam than for the lower frequencyaccelerator analyzer. Prior art analyzers have a time resolution, whichis the smallest measurable time interval over which the analyzer candistinguish between the intensity, i.e., the varying number of chargedparticles, at adjacent points along the beam, on the order of 30picoseconds. The modern highcurrent linear accelerators require ananalyzer which is capable of operating at approximately 2 picosecondstime resolution.

In addition to the inadequacies of the prior art analyzer in terms oftime resolution, transmission of highspeed signals, associated with thebeam, from the accelerator to the analyzer by conventional means isextremely difficult. Transmission of these high-speed sig- 1 nals fordistances on the order of 50 feet or more by coaxial cable is subject togreat induced error due to the difficulties of handling wire carriedhigh frequency signals.

It is therefore an object of this invention to provide a particle beamanalyzer operable in the 2 picosecond time resolution range.

Another object of this invention is to provide a particle beam analyzeroperable at a distance from the accelerator.

SUMMARY OF THE INVENTION A device is provided for analyzing a particlebeam developed by an accelerator having an RF driving signal. Theparticle beam is directed so that is passes through a transparentmedium, thereby developing light of intensity proportional to theintensity of the particle beam. A photocathode is aligned to detect thelight and generate an electron beam traveling along a longitudinal axisand of intensity proportional to the intensity of the light beam. The RFdriving signal is coupled via phase-varying means to X-axis and Y-axisdeflection systems, with the signal applied to the X-axis deflectionsystem being constantly maintained 90 out of phase with the signalapplied to the Y-axis system.

The electron beam is directed to pass through the X and Y deflectionsystems and thereby precesses about the longitudinal axis to describe acircular pattern in a plane perpendicular to the longitudinal axis. Theprecessing beam may be focused and the point of convergence made toimpinge on a fluorescent screen, thereby illuminating a circular traceof particular diameter. A mask having a slit therethrough is coupled tothe fluorescent screen so that an arc of the circular trace traversesthe slit. A photomultiplier tube is positioned directly opposite theslit so that it detects the intensity of that portion of the beamtraversing the slit. As the phase of RF driving signal applied to thedeflecting electrode is varied from 0 to 360, the photomultiplier tubedevelops a signal representative of the intensity of the trace at eachphase variation. An alternate embodiment of the invention provides aslit which is positioned to intersect the circular trace of the electronbeam. The slit is aligned so that the electron beam passing through theslit is directed to and the point of convergence impinges on the dynodeof an electron multiplier tube which, as the phase of the RF signalapplied to the deflecting electrodes is varied from 0 to 360, develops asignal corresponding to the intensity of the electron beam at eachvariation of phase.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic of thehigh-speed particlebeam analyzer;

FIG. 2 is a view of mask-screen-trace arrangement;

FIG. 3 is a partial schematic of an alternate embodiment of theanalyzer; and

FIG. 4 is a set of curves of the output of a particle beam analyzer.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. I, there isshown a schematic of a linear accelerator and particle beam analyzer. Alinear accelerator is a device in which charged particles gain in energyby the action of oscillating electromagnetic fields. A particle beamdeveloped in the accelerator is composed of a series of particle buncheswith the frequency at which the bunches are generated corresponding tothe frequency of oscillation of the electromagnetic fields. Forillustrative purposes, the linear accelerator l0 depicted in FIG. 1 isof the drift tube variety. Any other type of linear accelerator havingan RF driving signal is appropriate to practice the invention. Anotherexample of such an accelerator is the traveling wave linear accelerator.Linear accelerator 10 includes a series of drift tube electrodes l1, l2,l3, l4 and 15, which are coupled to an RF oscillator 18, via leads l6and 17. The RF oscillator 18 applies an RF signal to the electrodes.Charged particles are developed by particle source 20 and are thenaccelerated by linear accelerator 10 with a resultant beam from thelinear accelerator 10 consisting of a series of pulses or bunches ofparticles. Each pulse will be similar to every other pulse due to thecharacteristics of the accelerator so that errors will be repeated ineach pulse. It is desirable to detect the presence of these bunches andhow closely in time the particles are generated within each bunch todetermine if the accelerator is functioning properly. It is for thispurpose that a particle beam analyzer is necessary.

The charged particle beam 23 developed by linear accelerator l0 andcontained in an evacuated beam guide 24 is directed to pass through aquartz bead 25 outside guide 24. When an electrically charged particleis made to pass through a transparent medium, such as a quartz bead, ata velocity in excess of the speed of light in that transparent medium,Cherenkov radiation occurs at a light intensity proportional to thequantity of particles of the incident beam. Therefore, when the particlebeam 23 developed by linear accelerator is made to pass through quartzbead 25, light of intensity proportional to the quantity of particles inparticle beam 23 is generated. The quartz bead is positioned at thefocal point of parabolic reflector 28, and guide 24 is provided with analuminum window 26 so that the particle beam 23 can be extracted fromguide 24 and directed to the quartz bead where light beam 29 isgenerated and directed as shown. Note that any transparent medium havingthe properties described above will produce this Cherenkov radiationeffect.

The light beam 29 can be used to transmit the signal associated with thecharged particles over long distances without the distortion of thesignal associated with longdistance transmission of high-speed signalsby cable. This allows the analyzer to be positioned at relatively largerdistances from the accelerator than the analyzers requiring cable feeds.

Light beam 29 is directed towards an analyzer tube 30, the elements ofwhich may be found in a cathode ray tube. The analyzer tube 30 isprovided with a photocathode upon which light beam 29 is directed toimpinge. A photocathode is an electrode which includes a photoreactiveelement from which electrons are emitted due to the incidence ofradiation thereupon. Thus, when light beam 29 impinges upon photocathode35, photocathode 35 serves to generate electrons in a quantity orintensity proportional to the intensity of incident light beam 29. Theseelectrons are formed into a beam and focused by electron gun 36.Electron gun 36 includes those elements normally found in a cathode raytube electron gun such as an accelerating grid 37 and focusing anode 38.The electron beam formed by electron gun 36, if undeflected, can beconsidered to be traveling longitudinally along a Z-axis, converging ata particular point along the Z-axis. After the electron beam is formedand focused by electron gun 36, it is directed to pass through thehighfrequency deflection system 40, which acts to deflect the beam andthus the point of convergence from the Z-axis. Deflection system 40includes a pair of electrodes 41 and 42 for deflecting the electron beamfrom the Z-axis in the X direction and a pair of electrodes 44 and 45for deflecting the electron beam in the Y direction.

The output from RF oscillator 18 is amplified by amplifier 49 andcoupled to a variable phase shifter 50. Variable phase shifter 50 is adevice which is capable of varying the phase of an A-C signal from 0 to360 from its initial phase and of developing an output indicating thevalue of the amount of the phase shifting in terms of a variableamplitude D-C signal. X-axis deflecting cathodes 41 and 42 are coupledto the RF oscillator 18 via variable phase shifter 50 by leads and 56 sothat, as the phase of the RF signal is varied by phase shifter 50, thephase of the A-C signal applied to the X-axis deflecting electrodes iscomparably varied. Similarly, Y-axis deflecting electrodes 44 and 45 arecoupled to the RF oscillator 18 via variable phase shifter 50. However,the signal applied to Y-axis defleeting electrodes 44 and 45 via leads57 and 58 is maintained constantly 90 out of phase with the signalapplied to the X-axis deflecting electrodes 41 and 42 by 90 phaseshifter 60. The effect of applying identical A-C signals 90out of phaseto a deflection system including X-axis deflecting electrodes and Y-axisdeflecting electrodes is to induce the electron beam passing throughdeflection system 40 to precess about the Z- axis along which it wouldotherwise travel, thereby causing the point of convergence to describe acone about the Z-axis. Since the electron beam and the deflectionsignals ultimately derive from the same A-C source, RF oscillator 18,they are synchronized and as the point of convergence traverses ortraces one complete circle about the Z-axis it is also tracing one cycleof the RF signal.

A beam target system is provided to allow the cir cular trace of thepoint of convergence to be analyzed. In this embodiment, the beam targetsystem 65 includes a phosphorus screen 67 positioned so that the pointof convergence of the electron beam coincides with the screen at allnecessary deflections. As the point of convergence describes a circularpath it traces a circular fluorescent pattern upon phosphorus screen 67.The light output or intensity of any region of the fluorescent screen 67is proportional to the number of electrons of the beam bombarding thatregion at a particular instant. The number of electrons at any pointalong the beam is determined by the level of emissions from thephotocathode. Thus, it is apparent that the circular trace on the screenreflects the intensity and frequency of occurrence of the bunches of theparticle beam developed by the accelerator.

As shown in FIG. 1 and FIG. 2, an opaque screen or mask 68, which may beof tape, is applied to phosphorus screen 67. The mask includes a slit 69which is positioned so that an arc of circular trace 70 on screen 67traverses slit 69. With no phase variation applied to the RF signal byvariable phase shifter 50, at the beginning of each cycle of the RFsignal the point of convergence will be at a particular point C onscreen 67. As the RF signal goes through one entire cycle, the point ofconvergence will trace a circular path, always returning to begin eachcycle at .point C. If the phase of the RF signal applied to the X and Yaxis deflecting electrodes is varied a particular value between 0 and360 from its initial phase by variable phase shifter 50, the point ofconvergence will begin each cycle at a different point such as point D.The shift from C to D will be equal in angular degrees around the circleto the degrees of phase shift in the RF signal. Correspondingly, thatportion of the arc of trace 70 which traverses slit 69 will also changea like number of degrees. Therefore, as variable phase shifter 50 isvaried a particular value between 0 and 360 from its initial phase, thelocus of points describing trace 70 is shifted an equal amount, and theportion of the arc of trace 70 traversing slit 69 is also shifted anequal amount from the previous position. Photomultiplier tube ispositioned directly facing slit 69 so that light passing through slit 69impinges upon the photocathode of photomultiplier tube 75.

The width A of slit 69 determines the length of arc of trace 70 whichwill illuminate photomultiplier tube 75. Width A must be large enough topermit enough light to produce a suitable signal from photomultipliertube 75, but small enough for high resolution to allow examination ofthe smallest portion of the trace as possible for greater accuracy andless smearing caused by light from adjacent areas of the trace. A rangeof 0.25 mm to l mm is believed suitable for width A. Therefore, theoutput of photomultiplier tube 75, opposite slit 69, will beproportional to the intensity of the fluorescent arc of trace l0appearing at slit 69. As variable phase shifter 50 is varied over theentire range from 0 to 360, the output of photomultiplier tube 75 willvary according to the intensity of the are over that range.

An output signal is developed by variable phase shifter 50, indicatingat what phase between 0 and 360 the shifter is operating at thatinstant. This signal may be a DC signal which varies in amplitude toindicate the applicable phase. The output signal of variable phaseshifter 50 is coupled to one input of X-Y recorder 76. X-Y recorder 76is a device which plots on some form of chart the relationship betweentwo variables. The output of photomultiplier tube 75 is coupled to theother input of X-Y recorder 76. Remembering that each pulse of particlesfrom the accelerator will be similar to every other pulse of theaccelerator during one accelerator run, it can be seen that at anyparticular phase shift the output from photomultiplier tube 75 willremain substantially constant. As noted before, as variable phaseshifter 50 varies from 0 to 360, the output of photomultiplier tube 75varies accordingly, so that the plot display of X-Y recorder 76 willshow the variation in the amplitude of the signal from photomultipliertube 75 compared to the corresponding variation in the output signalfrom variable phase shifter 50. In effect, such a plot will representthe varying number of particles in a pulse from the accelerator over onecycle or timed period of the applied RF driving signal which at 1300megahertz means one cycle takes approximately 770 picoseconds. Thus, achange of 1 caused by variable phase shifter 50 will correspond to about2 picoseconds on the X-Y plotter. The output signal of the variablephase shifter 50 thus serves as the time base for the X-Y recorderdisplay.

Referring to FIG. 3, there is shown an alternate embodiment for targetsystem 65. The system of FIG. 1 re quires that the electron beamdeveloped by electron gun 36 be converted to a light beam before it isdetected by photomultiplier tube 75. It is the purpose of the embodimentof FIG. 3 to avoid this transformation and to directly detect theelectron beam. This is accomplished by allowing the electron beam tostrike the first dynode 77 of an electron multiplier tube 78 afterhaving passed through a slit 80 of mask 81, thus eliminating the secondconversion of electrons to light. Mask 81 is mounted on the casing 82which supports the electron gun and deflecting system and is aligned sothat, as the point of convergence of the electron beam describes acircle, that circle will traverse slit 80 with the point of convergenceimpinging upon the first dynode 77 of electron multiplier tube 78. Thewidth B of slit 80 is subject to the same size limitations as width A inFIG. 2. The signal developed by electron multiplier 78 is then coupledto the X-Y recorder by lead 83 as in the embodiment of FIG. 1.

Referring to FIG. 4, there is shown a representative set of plots whichmight be developed by X-Y recorder 76. Curve 85 represents the idealelectron beam pulse developed by a high-current accelerator having asharp rise and decay time and distinct amplitude peak indicatinggeneration of particleswithin each bunch closely together at one pointin time. Curve 86 represents an electron beam pulse from an acceleratornot functioning properly in that the pulse is spread out and notproperly bunched.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A device for analyzing a charged particle beam developed by anaccelerator having an RF driving signal, comprising:

a medium through which the particle beam passes at a speed sufficient toemit light from said medium by Cherenkov radiation of intensityproportional to the intensity of the particle beam;

means for developing an electron beam having an intensity proportionalto the intensity of said light and including a photocathode positionedto receive said light and responsive thereto to emit electrons to formsaid electron beam, means for directing said electron beam along aZ-axis and focussing said electron beam at a point, and X-axis and Y-axis deflection means for deflecting said focussed electron beam fromsaid Z-axis;

phase varying means coupled to the accelerator and to each of saiddeflection means and being responsive to the RF driving signal todevelop first and second signals of equal frequency to said RF drivingsignal, said phase varying means for applying said first signal to oneof said deflection means at a particular phase shift varying between 0and 360 from the phase of the RF driving signal and for applying saidsecond signal to the other of said deflection means at a phaseconstantly 90 out of phase with said first signal so that said electronbeam precesses about and said point describes a circular path about saidZ-axis;

a detector responsive to the quantity of electrons incident along aparticular are of said circular path to develop an output signalproportional to the intensity of said electron beam; and

recording means coupled to said phase varying means and to said detectorto record the value of said output signal with respect to saidparticular phase shift of said first signal applied to one of saiddeflection means.

2. The device of claim 1 wherein said detector includes a fluorescentscreen positioned so that said point impinges upon said screen toproduce a circular trace upon said screen, a mask having a slittherethrough coupled to said screen so that an arc of said circulartrace traverses said slit, and a photomultiplier tube facing said slitand responsive to the fluorescence of said arc of said trace passingthrough said slit to develop a tube output signal proportional to theintensity of said fluorescence.

3. The device of claim 2, wherein said phase varying means develops aphase output signal corresponding to the value of said particular phaseshift, and said recording means includes an X-Y recorder coupled to saidphase varying means and said detector, said X-Y recorder beingresponsive to said tube output signal and to said phase output signal tocontinuously display the intensity of the charged particle beam withrespect to one cycle of the RF driving signal.

4. The device of claim 2, wherein said slit is between 0.25 mm and l mmwide.

5. The device of claim 1 wherein said detector includes a mask having aslit therethrough positioned so that an arc of said circle described bysaid point travarying means and said detector, said X-Y recorder beingresponsive to said tube output signal and to said phase output signal todisplay the intensity of the charged particle beam with respect to onecycle of the RF driving signal.

7. The device of claim 5, wherein said slit is between 0.25 mm and 1 mmwide.

1. A device for analyzing a charged particle beam developed by anaccelerator having an RF driving signal, comprising: a medium throughwhich the particle beam passes at a speed sufficient to emit light fromsaid medium by Cherenkov radiation of intensity proportional to theintensity of the particle beam; means for developing an electron beamhaving an intensiTy proportional to the intensity of said light andincluding a photocathode positioned to receive said light and responsivethereto to emit electrons to form said electron beam, means fordirecting said electron beam along a Z-axis and focussing said electronbeam at a point, and X-axis and Y-axis deflection means for deflectingsaid focussed electron beam from said Zaxis; phase varying means coupledto the accelerator and to each of said deflection means and beingresponsive to the RF driving signal to develop first and second signalsof equal frequency to said RF driving signal, said phase varying meansfor applying said first signal to one of said deflection means at aparticular phase shift varying between 0* and 360* from the phase of theRF driving signal and for applying said second signal to the other ofsaid deflection means at a phase constantly 90* out of phase with saidfirst signal so that said electron beam precesses about and said pointdescribes a circular path about said Z-axis; a detector responsive tothe quantity of electrons incident along a particular arc of saidcircular path to develop an output signal proportional to the intensityof said electron beam; and recording means coupled to said phase varyingmeans and to said detector to record the value of said output signalwith respect to said particular phase shift of said first signal appliedto one of said deflection means.
 2. The device of claim 1 wherein saiddetector includes a fluorescent screen positioned so that said pointimpinges upon said screen to produce a circular trace upon said screen,a mask having a slit therethrough coupled to said screen so that an arcof said circular trace traverses said slit, and a photomultiplier tubefacing said slit and responsive to the fluorescence of said arc of saidtrace passing through said slit to develop a tube output signalproportional to the intensity of said fluorescence.
 3. The device ofclaim 2, wherein said phase varying means develops a phase output signalcorresponding to the value of said particular phase shift, and saidrecording means includes an X-Y recorder coupled to said phase varyingmeans and said detector, said X-Y recorder being responsive to said tubeoutput signal and to said phase output signal to continuously displaythe intensity of the charged particle beam with respect to one cycle ofthe RF driving signal.
 4. The device of claim 2, wherein said slit isbetween 0.25 mm and 1 mm wide.
 5. The device of claim 1 wherein saiddetector includes a mask having a slit therethrough positioned so thatan arc of said circle described by said point traverses said slit, andan electron multiplier tube facing said slit and responsive to said arcof said trace to develop a tube output signal proportional to theintensity of said beam.
 6. The device of claim 5, wherein said phasevarying means develops a phase output signal corresponding to the valueof said particular phase shift, said recorder means includes an X-Yrecorder coupled to said phase varying means and said detector, said X-Yrecorder being responsive to said tube output signal and to said phaseoutput signal to display the intensity of the charged particle beam withrespect to one cycle of the RF driving signal.
 7. The device of claim 5,wherein said slit is between 0.25 mm and 1 mm wide.